Personally I'm a bigger fan of Arena. It's not super new player friendly, but of the two, it's definitely friendly-er, imo, and you can play completely for free. Also, I'm a big fan of the Best-of-1 format being a thing on Arena. Makes playing much less of a time commitment, which is really nice.
The only thing about Arena that bugs me right now is Alchemy, which isn't something that's necessarily a problem with everybody, but is pretty hard to get away from, unfortunately.